lm2577sx-adj National Semiconductor Corporation, lm2577sx-adj Datasheet - Page 19

• Manufacturer Part Number: lm2577sx-adj
• Description: Simple Switcher Step-up Voltage Regulator
• Manufacturer: National Semiconductor Corporation

Application Hints
3. Output Voltage Selection (R1 and R2)
This section is for applications using the LM1577-ADJ/
LM2577-ADJ. Skip this section if the LM1577-12/LM2577-12
or LM1577-15/LM2577-15 is being used.
With the LM1577-ADJ/LM2577-ADJ, the output voltage is
given by
Resistors R1 and R2 divide the output down so it can be
compared with the LM1577-ADJ/LM2577-ADJ internal
1.23V reference. For a given desired output voltage V
select R1 and R2 so that
4. Input Capacitor Selection (C
The switching action in the step-up regulator causes a trian-
gular ripple current to be drawn from the supply source. This
in turn causes noise to appear on the supply voltage. For
proper operation of the LM1577, the input voltage should be
decoupled. Bypassing the Input Voltage pin directly to
ground with a good quality, low ESR, 0.1 µF capacitor (leads
as short as possible) is normally sufficient.
Cornell Dublier — Types 239, 250, 251, UFT,
300, or 350
Nichicon — Types PF, PX, or PZ
Sprague — Types 672D, 673D, or 674D
United Chemi-Con — Types LX, SXF, or SXJ
FIGURE 11. Aluminum Electrolytic Capacitors
Recommended for Switching Regulators
P.O. Box 128, Pickens, SC 29671
[phone]
927 East Parkway,
Schaumburg, IL 60173
[phone]
Box 1, Sprague Road,
Lansing, NC 28643
[phone]
9801 West Higgins Road,
Rosemont, IL 60018
[phone]
V
OUT
= 1.23V (1 + R1/R2)
(Continued)
IN
)
OUT
,
19
If the LM1577 is located far from the supply source filter
capacitors, an additional large electrolytic capacitor (e.g.
47 µF) is often required.
5. Diode Selection (D)
The switching diode used in the boost regulator must with-
stand a reverse voltage equal to the circuit output voltage,
and must conduct the peak output current of the LM2577. A
suitable diode must have a minimum reverse breakdown
voltage greater than the circuit output voltage, and should be
rated for average and peak current greater than I
and I
in switching regulators. Their low forward voltage drop allows
higher regulator efficiency than if a (less expensive) fast
recovery diode was used. See Figure 12 for recommended
part numbers and voltage ratings of 1A and 3A diodes.
BOOST REGULATOR CIRCUIT EXAMPLE
By adding a few external components (as shown in Figure
13), the LM2577 can be used to produce a regulated output
voltage that is greater than the applied input voltage. Typical
performance of this regulator is shown in Figure 14 and
Figure 15. The switching waveforms observed during the
operation of this circuit are shown in Figure 16.
(max)
V
100V
20V
30V
40V
50V
OUT
D(PK)
. Schottky barrier diodes are often favored for use
FIGURE 12. Diode Selection Chart
MBR120P MBR320P
MBR130P MBR330P
MBR140P MBR340P
MBR150
11DQ03
11DQ04
11DQ05
1N5817
1N5818
1N5819
1A
Schottky
MBR350
31DQ03
31DQ04
31DQ05
1N5820
1N5821
1N5822
3A
MUR105
HER102
MUR110
1N4933
1N4934
10DL1
Fast Recovery
1A
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HER302
MR851
MR831
30DL1
LOAD(max)
3A